Britain has announced that it is expelling 23 Russian diplomats. It's the single biggest expulsion since the Cold War. Prime Minister Theresa May announced the move today. It follows a high-profile poisoning of a former Russian spy and his daughter on British soil. Theresa May blamed the Russian government for that attempted murder, and she gave Russian officials a deadline to provide some kind of explanation. They did not. And today, May kicked their diplomats out of the country.

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PRIME MINISTER THERESA MAY: Through these expulsions, we will fundamentally degrade Russian intelligence capability in the U.K. for years to come. And if they seek to rebuild it, we will prevent them from doing so.

MARTIN: NPR's Joanna Kakissis is following this from London and joins me now on the line. Joanna, just remind us the backdrop of this and why it's happening.

JOANNA KAKISSIS, BYLINE: So, Rachel, Sergei and Yulia Skripal were found on March 4 in the southern English city of Salisbury. That's where they lived. That's where Sergei lived. He's a former Russian spy. And Yulia was his daughter. Yulia's his daughter. She was visiting from Moscow. They were found slumped on a park bench. And it turned out that they had been poisoned by a military-grade nerve agent that had been produced in the Soviet Union.

This alarmed a lot of people. It turned out that hundreds of other people in Salisbury may have been exposed to the nerve agent. And this set off a chain of events in the U.K. saying, well, why is this happening? Especially in the backdrop - there's a backdrop of the U.K. having had several suspicious murders of Russian exiles here - who have sought refuge here. So this has alarmed the government.

MARTIN: So Theresa May connected this to the Russian government. She said, you need to explain what has happened. They didn't. Now she's made this expulsion. Is this the end of it? I mean, we can imagine Russia might retaliate. But is the U.K. talking about doing anything else, sanctions or any kind of - any other kind of censure?

KAKISSIS: So there has been some talk about Russia Today - or RT - losing its broadcasting license as a result of this. And Russia has been - has talked back and said, wait a minute. If that happens, well, then we're going to expel your correspondents, British correspondents from Russia. So Russia is not in the mood to negotiate or to, you know stand back on this. They say they will also retaliate if something like that happens.

MARTIN: And Russia, I mean, have they said anything about the original accusation that they are somehow connected to this poisoning?

KAKISSIS: Well, Russia has dismissed all links to this. They say they have no involvement. They did not want to speak to Theresa May after her government summoned the Russia - Russia's ambassador to the U.K. to explain this. They said, we're not going to answer any questions about this because we weren't involved. And anyway, we'd like to take a look at this nerve agent to see if it has anything to do with anything. I mean, we don't want to talk at all. So they have - again, they have not been in the mood to talk with Theresa May about anything and deny all involvement.

MARTIN: All right. NPR's Joanna Kakissis reporting from London this morning on the fact that Theresa May, the prime minister there, has expelled 23 Russian diplomats from the U.K. Joanna, thanks so much. We appreciate it.